1. The Field of the Invention
This invention relates generally to a method for the extraction of organic liquid contaminates from an aqueous sample. This invention is directed to the surprising ability of hydrophobic sheet or ribbon tape made of a hydrophobic polymer to facilitate the extraction of liquid organic contaminates from an aqueous sample. This invention further provides a method which facilitates the analysis of hazardous or toxic waste sites in the field, i.e. outside a laboratory environment.
2. The Background Art
In general, historical methods for the extraction of organic contaminates from an aqueous sample involved the addition of a suitable amount of a non-polar organic solvent such as methylene chloride or octane to the aqueous sample from which contaminates were to be extracted. Because the added organic solvent is non-polar and water is polar the two liquids are immiscible. This opposition in polarity and inherent immiscibility causes the two liquid systems to separate into two layers, much like the separation of oil and water into two layers. A typical separation would place both liquid phases in a separatory funnel which allows for the separation of the two phases by selectively removing one layer. The two phases are shaken or vigorously stirred together to provide sufficient contact between the two phases. The contact between the two phases allows the components of the mixture to migrate into the phase which resembles the contaminate's polarity. Because most contaminates of interest are non-polar, the non-polar organic phase tends to extract the contaminates from the polar aqueous phase into the organic phase. The organic phase including the contaminates can then be separated from the water phase. Once the organic phase is separated from the aqueous phase and had all water removed therefrom it is analyzed for contaminate content. Frequently this analysis has as its goal the determination of the presence or absence of a specific organic contaminate, such as the presence of polyaromatic hydrocarbons or polychloro phenols. Moreover, these organic contaminates may be present in only minor amounts when compared to the volume of the sample to be tested. Because the organic contaminate is present in only minor amounts, it is frequently desirable to concentrate the sample many hundred-fold to facilitate the observation of the contaminate. Indeed where the contaminate is present in parts per million (ppm) or parts per billion(ppb) it would likely remain undetected unless the contaminate concentration were increased.
The process of concentration often involves an evaporative process. This process involves heating of the sample whereby the majority of the solvents are driven off, leaving the contaminates in the evaporative reservoir. However, this process relies on the contaminate having both a higher boiling point and a lower vapor pressure than the organic solvent used to effectuate the contaminate's extraction. If the contaminate has a higher boiling point than the solvent, the evaporative process results in the evaporation of the solvent, resulting in a concentrated mixture containing the contaminates. But where the contaminate has a lower boiling point than the solvent, the contaminate will evaporate with the solvent and be discarded, thus destroying the reliability of the extraction. Additional concerns exist where the extraction solvent forms a binary or tertiary vapor state complex with one or more of the contaminates. When such a vapor phase interaction occurs, the involved contaminates co-distill with the solvent, even where the compounds involved have different vapor pressures and boiling points. The co-distillation would result in one or more contaminates being distilled with and disposed with the solvent. Such an interaction would likewise defeat the goal of the extraction and concentration procedure because unknown contaminates would be lost or disposed of prior to detection providing a false negative regarding the presence of the contaminate in the sample.
Accordingly, the process of concentration through evaporation is fraught with difficulties and concerns. These concerns are more prominent in the area of waste analysis where the goal is the elucidation of contaminates within an aqueous sample. Often in such samples, the contaminates and corresponding concentrations within the sample are unknown. Therefore, any method of analysis which increases the potential for loss of a contaminate renders the result of the analysis questionable. In order to circumvent these concerns, methods of extraction and analysis have been developed which are able to extract very small quantities of contaminates from within a sample.
One method of extraction which has been investigated involves the injection of a sample containing an unknown into a liquid chromatography column (LC). The LC facilitates the extraction of the organic unknowns from the aqueous sample. The LC further allows for the separation of the organic unknowns into individual compounds for individual analysis. The LC requires, however, substantial amounts of organic solvents, increasing the quantity of organic waste. The LC also causes an increase in the dilution of the organic contaminates after extraction and requires bulky instrumentation. Moreover, LC extraction is a time intensive process not suited for the rapid extraction of organic contaminates from an aqueous sample. Neither can the LC method be used outside of a laboratory setting at a field site for the extraction of contaminates from waste at a site.
It has been proposed that an efficient extraction technique would maximize the extraction of contaminates from an aqueous sample and minimize the organic solvent waste produced by the extraction. One intensely investigated field, solvent extraction-flow injection, involves the careful formation of a wetting film on the interior surface of a capillary tube with an hydrophobic organic solvent. Solvent extraction-flow injection (SE-FI) has become an essential analytical tool for a direct method of analysis or as a separation and pre-concentration step for further analysis of unknown samples. The SE-FI method is a form of liquid-liquid extraction where the contaminates within the aqueous phase are washed out or extracted from the aqueous phase through the interaction of an organic solvent. However, the SE-FI system allows for LC type extraction but with the use of less solvent.
Classical SE-FI involves the following operations: (1) segmentation with an immiscible organic solvent, (2) equilibration in an extraction coil, and (3) separation of the organic phase from the aqueous phase for determination. The most critical aspects of the conventional SE-FI system are the segmentation and phase separation, with respect to their influences on the reproducibility, reliability, stability and overall dispersion of the sample zone. These processes have been extensively studied and many refinements of segmentation and phase separation of solvent extraction in flow systems have been made. The SE-FI method relies on the formation of discrete segments of liquid corresponding to the aqueous and organic portions of the sample within a small diameter tube. The extraction mechanism of the SE-FI systems to date is basically the same as that of batch LC separation procedures. Two immiscible phases are brought together and mixed with each other causing the extraction of the contaminate from the aqueous system into the organic phase and an analysis of the contaminates is then made with or without phase separation. However, the SE-FI method relies on the same LC type liquid-liquid extraction to separate the contaminates from the aqueous sample. The tubing serves only as a support for the segments where the extraction which occurs between the segments is determined by the two liquids used to facilitate the extraction, not the chemical make-up of the tubing.
Contrasted to the SE-FI method, in a segmented flow system the reproducibility and reliability of the results depend on the repeatability of flow rates and segmenter and/or the quality of the phase separator. This type of extraction system requires the continuous formation of organic and aqueous phase segments within the extraction tube. Each aqueous segment is extracted by the preceding and following organic phases. This technique provides for twice the extraction power of a classical SE-FI system, but with more waste produced. Because the aqueous and organic phases must be flowing continuously during the whole process, the consumption of organic solvents and other chemicals is relatively high which produces more laboratory waste and precludes such methods for field use. Further, the concentration factors of these systems are generally small (&lt;20) because the ratio of the flow rates of the aqueous phase containing the contaminates and the organic phase cannot be increased indefinitely.
The film-forming characteristics of organic solvents on Teflon.TM. tubing walls have been used to produce another SE-FI/sequential injection system. The organic solvent, aqueous sample solution and eluting solution are sequentially aspirated into the extraction coil. The extractable contaminates in the aqueous solution were extracted into the organic wetting film formed on the wall of the extraction coil and then eluted by microliter volumes of an eluting solution; concentration factors as high as 150 can be achieved. The need for a segmenter and a phase separator are eliminated and both the consumption of organic solvents and the laboratory wastes are reduced. However, it is still essential that the organic film be maintained on the tube wall for extraction of the organic contaminates to occur. Regardless of method and procedure of use, the extraction is accomplished through the liquid-liquid phase interaction between the polar and non-polar phases. It is the energetic motivation for the non-polar contaminates to migrate out of the polar phase, into the non-polar phase which accomplishes the extraction. Indeed the surface area of the liquid-liquid interface is of crucial importance because extraction can only occur at the polar/non-polar boundary. The greater the area of polar/non-polar boundary interaction the more efficient the migration of contaminates from the polar to the non-polar layer.
In two immiscible phase liquid flows such as are found in an SE-FI extraction coil the solvent displaying a greater affinity to the tubing material will cover the tubing inner walls with a very thin film of relatively stationary nature. In hydrophobic tubing such as Teflon.TM. or other non-polar polymers, the organic solvent acts as the film-forming phase, and the aqueous segments will be surrounded by the film of organic solvent. The driving force for the film formation is the minimization of the interfacial energy at the solid/liquid interface, which is determined by the relative magnitudes of the surface tension of the inner tubing wall surface in contact with the liquids and the interfacial tension of the liquids. There is no exact theory for the film formation in SE-FI systems but many agree the film is formed to minimize polar/non-polar interactions between the tubing wall and the aqueous phase.
The formation of the wetting film increases the mass transfer speed and extraction efficiency because there are two basically different transport mechanisms in the extraction coil: transport through the vertical interfaces between the two phases (axial extraction) and transport through the wetting film on the tubing wall (radial extraction). However, the existence of the wetting film, which bridges the adjacent segments of organic solvent, allows transport of the extractable components from one organic segment to subsequent organic segments. Hence the presence of the film results in peak dispersion within the extraction coil. The thicker the film, the greater is the peak broadening. Therefore, in classical SE-FI, the prevailing philosophy is to minimize the band broadening by using organic solvents that yield thin wetting films. Unfortunately this technique also limits the interacting surface area of the non-polar film with the contaminate containing polar phase. Moreover, peak broadening can be further minimized through the increased formation of alternating polar non-polar segments along the inner wall thereby precluding the migration of organic contaminates between non-polar segments. This technique, however, further diminishes the surface area of polar/non-polar interaction, decreasing extraction efficiency.
The linear velocities in SE-FI and flow systems are generally low, and the prevailing flow pattern is laminar. The flow velocity near the tubing wall is zero, whereas it is twice the mean flow velocity in the center of the tubing. This severe discrepancy in flow adversely affects the extraction of contaminates from the center of the aqueous flow into the organic phase at the wall. Because the film on the inner wall is very thin, it forms part of a relatively stationary phase along the tubing wall and can only cause the desired extraction of contaminates where the contaminate contacts the organic phase. Because of the discrepancy in the flow velocities between the polar and non-polar phases, the extraction dynamics are adversely affected, decreasing the efficiency of extraction. The extractable components in the flowing aqueous solution can be extracted into the organic wetting film when they enter the diffusion zone of the two phases. The diffusion zone is the interface or point of contact between the two phases. Solutes in the central part of the aqueous flow will diffuse to the diffusion zone and be extracted by the subsequent section of the film. This type of extraction system requires a certain contact time for the extractable components in the aqueous flow to be extracted into the organic wetting film. The required period of time is determined by the contaminates contained within the sample. Where the contaminates are unknown, the required period of time is also unknown and may demand a prolonged contact period to ensure proper extraction. Additionally, the organic wetting film remains adhered to the inner portion of the tubing wall and requires an additional extraction step to remove the extracted contaminates from the wetting film.
Three approaches can be used to elute (wash out) the extracted components: (i) elution with another section of the same organic solvent forming the wetting film: (ii) elution with another organic solvent that can wash out both the extracted components and the wetting film; and (iii) back-extraction of the extracted components. The last two methods are more efficient than the first because the first method uses a new portion of the wall coating solvent to replace the wetting film on the tubing wall and there is a distribution equilibrium of the extracted components between the eluting solvent and the wetting film, which results in a loss of the extracted contaminates. Because the thickness of the film is of the order of a few tens of micrometers, back-extraction is fast and efficient. However, because the wetting film is so thin, the surface area of interaction between the two phases is also at a minimum, limiting the efficiency and speed of the extraction.
In a conventional SE-FI system, the extraction efficiency is influenced by the tubing length, inner diameter and geometry. It is assumed that all of these factors have some effects on the extraction efficiency of any proposed system. Further, the wetting film is the only organic phase available for the extraction of contaminates in the system. The volume of the organic film is much less than that of the segmented polar system. Therefore, the most critical aspect of these system is the extraction capacity and surface area of the wetting film. The geometry of the extraction tubing also has some effects on the extraction efficiency. However, regardless of coil shape, all extraction methods involve a tubular extraction geometry with the included problems associated with tubular systems. Furthermore all known systems require a sophisticated injection/segmentation system to allow for the creation of a stationary wetting film into which the contaminates will be extracted. Moreover all methods of the prior art require that the extracted contaminates be "washed" out of the extraction tube, often requiring substantial additional solvent. The increase in solvent output requires the use of an evaporative concentrator, thereby incurring the problems associated with such concentrators.
These and similar extraction methods require substantial hardware and technical expertise which preclude their use in a field extraction of contaminates. It would therefore be useful to have an extraction system which required only a minimum of laboratory paraphernalia. Furthermore it would be useful to have an extraction method which made use of a hydrophobic polymer which was not limited by tubular shape, thus providing increased efficiency and increased surface area of interaction because of the new shape